It is a known phenomenon that the modules of certain photovoltaic generators, particularly so-called thin film modules, only achieve their maximum lifetime if they are completely kept on negative or positive electric potential with regard to earth potential, particularly during their operation.
For the purpose of ensuring such an electric potential of the modules of all photovoltaic generators connected to an inverter, an offset voltage may be applied in a solar power plant to shift the potential center point of the photovoltaic generators with regard to a reference potential, particularly directly with regard to earth potential.
The offset voltage may be applied to a current carrying line via which all photovoltaic generators are connected to a DC input of the inverter. In a transformerless inverter, this shift of the potential center point of its DC input also has an effect on its AC output, and it will spread from there up to the DC input of further transformerless inverters connected in parallel. Correspondingly, with a transformerless inverter, the offset voltage may also be applied to a current carrying line which is connected to the AC output of the inverter.
European patent EP 2 136 449 B1 discloses a solar power plant including a DC voltage source in an offset path for shifting the potential center point of all photovoltaic generators connected. The offset path leads from one current carrying line at the primary side of a transformer to earth potential. The transformer provides a galvanic separation or isolation between several transformerless inverters connected in parallel and a grid terminal for an external AC power grid. When the transformer comprises a center point at its primary side, the offset path is connected to this center point. If the transformer comprises a delta configuration at its primary side, the offset path extends between one of the phase lines and earth potential.
The inverters are connected in parallel, and each inverter feeds electric power from one photovoltaic generator into one of three phase lines to the transformer. The offset voltage of the DC voltage source has the effect that the photovoltaic generators connected to the inverters are completely on positive electric potential. Further, a current surveying device of the DC voltage source can be used for detecting earth faults. In case of an isolation or earth fault, an increased current via earth potential flows through the offset path. In response to this increased current, a contactor is triggered which disconnects the inverter or the grid terminal from the transformer. A center point line extending between the offset path and the inverters is not disconnected here.
European patent EP 2 136 449 B1 does not refer to connecting the individual inverters to an internal AC power grid at the primary side of the transformer and having a shifted potential center point. The contactor which separates the current connection between the inverters and the transformer or between the grid terminal and the transformer is not suited for this purpose. Thus, it is assumed that the inverter in the solar power plant known from European patent EP 2 136 449 B1 is connected in the usual way, i.e. that the inverter is first synchronized with the internal AC power grid and that then all current carrying lines of the AC output of the synchronized inverter are galvanically connected to the internal AC power grid. Such a procedure is indicated as usual in European patent application publication EP 2 242 160 A1, for example.
A further power plant including a DC voltage source in an offset path for shifting the potential center point of connected photovoltaic generators is known from WO 2010/051812 A1. Here, one or more transformerless three-phase inverters are connected in parallel and each feed electric power from a photovoltaic generator into an external AC power grid via a common transformer providing a galvanic separation or isolation. The offset path is connected to the center point of the primary side of the transformer, independently of whether the center point is connected to the individual inverters or not. In this known circuitry arrangement, the height of the offset voltage which is provided by the DC voltage source is adjusted to the present output voltage of the photovoltaic generators. This output voltage is typically some hundred volts. To bring the photovoltaic generators completely on a positive or negative voltage with regard to earth potential, the offset voltage has to amount to half the output voltage of the photovoltaic generators. For this purpose, the offset voltage is variable in a range from 400 V to 500 V. WO 2010/051812 A1 does not refer to connecting the individual inverters to the internal AC power grid at the primary side of the transformer and having a shifted potential center point.
A measurement arrangement for measuring the isolation resistance of a living electrical device, like for example a photovoltaic generator, is known from DE 10 2006 022 686 A1. Here, two switches or a corresponding changeover switch are provided which each provide a current path between one of the two electric poles of the electrical device and earth potential. The current flowing through a shunt resistor in this earth connection path from one of the electric poles depends on a voltage between the two electric poles and is a measure of the isolation resistance of the respective other pole.
A method of connecting a photovoltaic generator to an input side DC voltage link of an inverter of a solar power plant is known from European patent application publication EP 2 228 893 A2. Here, the DC voltage link is preloaded prior to actually connecting it to the output voltage of the photovoltaic generator to avoid current peaks due to reloading or short-circuit currents.
There is still a need for a method of connecting a transformerless inverter in a solar power plant with a shifted center point which ensures a long lifetime of a controllable switch used for galvanically connecting the current-carrying lines of the output of the inverter to an internal AC power grid at an input side of a galvanic isolation.